HE had not expected to spend his 60th birthday in a hospital cardiac unit. R. J. Turner, a commercial real estate broker from Frederick County, Va., had planned a robust celebration. “I was going to finish my 10th marathon,” Mr. Turner said, “which isn’t bad for a guy my age.”

But near the start of the Marine Corps Marathon on Oct. 29, Mr. Turner raised an arm to wave at bystanders, and “everything went black.” Collapsing violently, he gashed his head, chipped a tooth and bit a deep hole in his bottom lip.

Mr. Turner, who had passed a stress test a year before, had just had a heart attack.

This has been an unusual season for the cardiac health of marathoners. After years in which almost no deaths were attributed to heart attacks at this country’s major marathons, at least six runners have died in 2006.

Two police officers, one 53, the other 60, died of heart attacks at the Los Angeles Marathon in March. The hearts of three runners in their early 40s gave out during marathons in Chicago in October, San Francisco in July and the Twin Cities in October. And at the same marathon where Mr. Turner was felled, another man, 56, crumpled near the 17th mile, never to recover.

Race fields have grown. In 2005, 382,000 people completed a marathon in the United States, an increase of more than 80,000 since 2000, according to marathonguide.com. Meanwhile, the risk of dying from a heart attack during a marathon is about 1 in 50,000 runners, said Dr. Arthur Siegel, the director of internal medicine at McLean Hospital in Belmont, Mass., and an assistant professor of medicine at Harvard.

But some physicians, including Dr. Siegel, an author of more than two dozen studies of racers at the Boston Marathon, wonder if there is more to the deaths than mathematical inevitability: Does racing 26.2 miles put a heart at risk?

A new study by Dr. Siegel and colleagues at Massachusetts General Hospital and other institutions is at least suggestive. Sixty entrants from the 2004 and 2005 Boston Marathon were tested before and after the race. Each was given an echocardiogram to find abnormalities in heart rhythm and was checked for blood markers of cardiac problems — in particular for troponin, a protein found in cardiac muscle cells. If the heart is traumatized, troponin can show up in the blood. Its presence can determine whether there has been damage from a heart attack.

The runners (41 men, 19 women) had normal cardiac function before the marathon, with no signs of troponin in their blood. Twenty minutes after finishing, 60 percent of the group had elevated troponin levels, and 40 percent had levels high enough to indicate the destruction of heart muscle cells. Most also had noticeable changes in heart rhythms. Those who had run less than 35 miles a week leading up to the race had the highest troponin levels and the most pronounced changes in heart rhythm.

The findings, published in the Nov. 28 issue of Circulation, a journal of the American Heart Association, were a surprise, and not least to the runners. None had reported chest pains or shortness of breath at the finish. All had felt fine, Dr. Siegel said (to the extent one can feel fine after pounding through 26.2 miles).

Within days, the abnormalities disappeared. But something seemed to have happened in the race. “Their hearts appeared to have been stunned,” Dr. Siegel said.

“Although the evidence is not conclusive, it does look like the Boston study is showing some effect on cardiac muscle,” said Dr. Paul D. Thompson, 59, the director of cardiology at Hartford Hospital in Connecticut, and an author of an editorial that accompanied the study. “It’s far too early to draw any conclusions,” he added. “We’d be seeing lots more bodies piling up if there were real lingering long-term cardiac damage” caused by running marathons.

“Over all, the evidence is strongly in favor of the idea that endurance exercise is helpful in terms of cardiac health,” said Dr. Thompson, who has run more than 30 marathons.

Photo

With him is Dr. Frederick C. Lough, director of cardiac surgery at George Washington University Hospital and himself a runner.Credit
Heather Bancroft/George Washington University

But questions do remain. Another new study, this one out of the University of Duisburg-Essen in Germany, showed completely unexpected results in a group of experienced middle-aged male marathoners. In the study, which was presented in November at a meeting of the American Heart Association, the subjects, each of whom had completed at least five marathons, underwent an advanced type of heart screening called an electron beam CT scan. Unlike echocardiograms or stress tests, electron beam CTs show the level of calcium plaque buildup or atherosclerosis in the arteries.

More than a third of the runners had significant calcium deposits, suggesting they were at relatively high risk for a heart attack. Only 22 percent of a control group of nonrunners had a comparable buildup.

The researchers scrupulously avoided suggesting that marathoning had caused the men to develop heart disease. (After all, running may have kept them alive when they would otherwise have keeled over years earlier.) But neither did the authors rule out the possibility that in some baffling way distance running had contributed to the men’s arterial gunk.

What worries Dr. Siegel and some of his colleagues is that marathons present an opportunity for silent symptomless heart disease to introduce itself abruptly. The pulsing excitement, the adrenaline, the unpleasant process of “hitting the wall” may trigger physiological changes that loosen arterial plaques, precipitating a heart attack, Dr. Siegel said.

His advice to runners with any history of heart trouble is “train for the race, getting the cardiac benefits of endurance exercise,” then watch the event on television.

The risk of going into cardiac arrest as a spectator, he said, is only about one in a million. (The applicable studies of spectators involved Super Bowl fans.)

Anyone considering joining the ranks of marathoners should undergo a full medical screening, with a visit to a cardiologist for those over 40, Dr. Siegel said. Spiral or electron beam CT scans are desirable (the cost can range from $250 to $850) and are covered by insurance if recommended by a physician.

Those with a family history of cardiac problems should be especially cautious. “You can’t outrun your genes,” Dr. Siegel said, a reality that marathon medical experts call the Jim Fixx effect, after the author of “The Complete Book of Running,” who died of a heart attack in 1984 at 52. His father had also died young.

Still, the majority of cardiologists remain avid fans of marathons. “It is an extraordinary event,” said Dr. Frederick C. Lough, the director of cardiac surgery at George Washington University Hospital in Washington. “But you have to respect that distance. It’s not something everyone necessarily should attempt.”

Dr. Lough, 57, was less than a block behind Mr. Turner when the older man collapsed. He interrupted his own race to help revive Mr. Turner and accompany him to the hospital, before completing the marathon. “It was a vivid reminder that running does not make anyone immune to heart disease,” Dr. Lough said.

Experts familiar with the new cardiac studies of marathoners urge caution and perspective. The numbers of people studied were small, the findings unexplained, and results have not yet been replicated.

Don’t use the studies, in other words, to justify parking yourself smugly on the couch. “There’s not yet in my opinion cause for alarm,” Dr. Thompson said. “I would still tell people, run.”

His words doubtless will cheer Mr. Turner. “You know the worst thing about almost dying?” he said. “That I didn’t finish.” After having had a stent installed in his heart to open an artery that was about 98 percent blocked, he’s now walking a mile a day and planning his comeback. “I want to get that 10th marathon in,” he said.

But not before he gets a full medical screening, including an electron beam CT scan.

Correction: December 21, 2006

An article on Dec. 7 about marathon running’s effects on the heart misstated the type of diagnostic machine used in a study by the University of Duisburg-Essen in Germany. It is an electron-beam CT scan, which uses electron beams to show the interior of coronary arteries, not a spiral CT scan, which uses X-rays to do so.

A version of this article appears in print on , on page G10 of the New York edition with the headline: Is Marathoning Too Much of a Good Thing for Your Heart?. Order Reprints|Today's Paper|Subscribe